System and method to generate electricity

ABSTRACT

A split heat recovery steam generator (HRSG) arrangement including a first HRSG coupled to a turbine and thereby receptive of a portion of the exhaust gases to deliver the portion of the exhaust gases to a compressor, a second HRSG coupled to the turbine and thereby receptive of a remaining portion of the exhaust gases, which includes an NOx catalyst and a CO catalyst sequentially disposed therein to remove NOx and CO from the exhaust gases and an air injection apparatus to inject air into the second HRSG between the NOx catalyst and the CO catalyst to facilitate CO consumption at the CO catalyst.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to gas turbine engines and,more particularly, to gas turbine engines using stoichiometric exhaustgas recirculation.

In gas turbine (GT) engines, compressed air and fuel are mixed togetherand combusted to produce high energy fluids that are directed to aturbine section where the fluids interact with turbine buckets togenerate mechanical energy, which can be employed to generate power andelectricity. In particular, the turbine buckets may rotate a shaft towhich an electrical generator is coupled. Within the electricalgenerator, the shaft rotation induces current in a coil electricallycoupled to an external electrical circuit. As the high energy fluidsleave the turbine section they can be redirected to a heat recoverysteam generator (HRSG) where heat from the fluids can be used togenerate steam for steam turbine engines and further power andelectricity generation.

With the combustion of the air and the fuel, however, emissions and/orpollutants, such as Carbon Monoxide (CO) and Oxides of Nitrogen (NOx),are produced and may be found in gas turbine engine emissions.Reductions of these pollutants are necessary to lessen the negativeenvironmental impacts of gas turbine engines and to comply withregulations. Often however reduction of pollutants is directly connectedto a loss of efficiency that impacts the economic value of electricitygeneration.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a split heat recovery steamgenerator (HRSG) arrangement to reduce emissions in exhaust gasesemitted from a turbomachine system to generate electricity including acompressor, a combustor fluidly coupled to and disposed downstream fromthe compressor and a turbine fluidly coupled to and disposed downstreamfrom the combustor to receive combustion gases and fluidly connected tothe compressor to receive cooling and leakage fluid is provided. Thearrangement includes a first HRSG coupled to the turbine and therebyreceptive of a portion of the exhaust gases to deliver the portion ofthe exhaust gases to the compressor, a second HRSG coupled to theturbine and thereby receptive of a remaining portion of the exhaustgases, which includes an NOx catalyst and a CO catalyst sequentiallydisposed therein to remove NOx and CO from the exhaust gases and an airinjection apparatus to inject air into the second HRSG between the NOxcatalyst and the CO catalyst to facilitate CO consumption at the COcatalyst.

According to another aspect of the invention, a system to generateelectricity is provided and includes a turbomachine. The turbomachineincludes a compressor, a combustor fluidly coupled to and disposeddownstream from the compressor and a turbine fluidly coupled to anddisposed downstream from the combustor to receive combustion gases andfluidly connected to the compressor to receive cooling and leakagefluid. The system further includes a first heat recovery steam generator(HRSG), coupled to the turbine and thereby receptive of a portion ofexhaust gases output from the turbine, the first HRSG being configuredto deliver the portion of the exhaust gases to the compressor, a secondHRSG, coupled to the turbine and thereby receptive of a remainingportion of the exhaust gases output from the turbine, which includes anNOx catalyst and a CO catalyst sequentially disposed therein to treatthe received exhaust gases and an air injection apparatus to inject airinto the second HRSG between the NOx catalyst and the CO catalyst tofacilitate CO consumption at the CO catalyst.

According to yet another aspect of the invention, a method of operatinga turbomachine system to generate electricity is provided and includespermitting a portion of turbomachine exhaust to be received in a firstHRSG for delivery to a compressor of the turbomachine and a remainingportion of the turbomachine exhaust to be received in a second HRSG inwhich NOx and CO in the GT engine exhaust are consumed, sensing whetherthe NOx and the CO consumption is substantially complete and, in anevent the NOx consumption is incomplete, modulating the relative amountsof the portion and the remaining portion of the turbomachine exhaustrespectively received in the first and second HRSGs and, in an event theCO consumption is incomplete, modulating an amount of air injected intothe second HRSG.

These and other advantages and features will become more apparent fromthe following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWING

The subject matter, which is regarded as the invention, is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a schematic illustration of a system with a turbine engine anda split heat recovery steam generator (HRSG) arrangement;

FIG. 2 is a schematic diagram of a controller for the system of FIG. 1;

FIG. 3 is a schematic diagram of an exhaust system for use with thesystem of FIG. 1; and

FIG. 4 is a flow diagram illustrating a method of operating the systemof FIG. 1.

The detailed description explains embodiments of the invention, togetherwith advantages and features, by way of example with reference to thedrawings.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1 and 2, a system 10 is provided to generateelectricity with reduced emissions. More particularly, the system 10 isprovided to generate electricity with little or no Oxides of Nitrogen(NOx) or Carbon Monoxide (CO) pollutant emissions. The system 10includes a turbomachine, such as a gas turbine (GT) engine 20, a splitheat recovery steam generator (HRSG) arrangement 60 having a first HRSG70 and a second HRSG 80 and an air injection apparatus 115. In the GTengine 20, a compressor 21 compresses exhaust recirculated from thesecond HRSG 80, which is supplied to a combustor 22 that is fluidlycoupled to and disposed downstream from the compressor 21. The combustor22 may include at least one or more of dry low NOx (DLN) componentsand/or late lean injection (LLI) components that serve to reduce NOxemissions. A turbine 26 is coupled to the compressor 21 and thecombustor 22 to receive combustion products from the combustor 22 andcooling and leakage fluid from the compressor 21.

An upstream compressor 30 is coupled to the shaft 25 such that therotation of the shaft 25 drives upstream compressor 30 operation bywhich inlet air 35 is compressed and then made deliverable to thecombustor 22 via conduit 36. Within the combustor 22, the compressedexhaust from the compressor 21 is mixed with air from the upstreamcompressor 30 and fuel, such as natural gas, synthetic gas (syngas)and/or combinations thereof The mixture is then combusted to producehigh energy fluids from which mechanical energy may be derived in theturbine 26. In particular, the mechanical energy derived from the fluidscauses the shaft 25 to rotate at high speed. An electrical generator 40is also coupled to the shaft 25 whereby the rotation of the shaft 25induces the electrical generator 40 to generate an electrical current.In an exemplary embodiment, the fluids are communicated to the first andsecond HRSGs 70 and 80 via conduits 23 and 24, respectively.

The first HRSG 70 is fluidly coupled to the turbine 26 and is therebyreceptive of a portion of GT engine exhaust, which is untreated, viaconduit 23. The first HRSG 70 is configured to exploit the heat of theGT engine exhaust as the GT engine exhaust moves through the first HRSG70 from a first inlet end thereof to a second end thereof in theproduction of steam from which additional power and/or electricity maybe produced. A stoichiometric exhaust gas recirculation (SEGR) conduit90 is coupled to the second end of the first HRSG 70 and serves todeliver the portion of the GT engine exhaust back to the compressor 21in an SEGR loop. The SEGR loop may further cool and treat the exhaustgases.

The second HRSG 80 is fluidly coupled to the turbine 26 and is therebyreceptive via conduit 24 of a remaining portion of the GT engineexhaust, which is at least initially also untreated. The second HRSG 80operates similarly as the first HRSG 70 in that the second HRSG 80 mayalso produce steam from which additional power and/or electricity may beproduced. In addition, the second HRSG 80 includes a first CO catalyst100, an NOx catalyst 110 and a second CO catalyst 120, which aresequentially disposed therein to provide stoichiometric treatment of theGT engine exhaust as the GT engine exhaust travels through the secondHRSG 80. In some cases, only the NOx catalyst 110 and the second COcatalyst 120 are provided in the second HRSG 80.

With the relative amounts of the portion of the GT engine exhaustpermitted to flow to the first HRSG 70 and the remaining portionpermitted to flow to the second HRSG 80 maintained at respectivepredefined levels, the GT 20 can be made to operate such that the GTengine exhaust is substantially free of oxygen (0 ₂). The GT 20 may betherefore positioned for application of the three way catalysts of thesecond HRSG 80 to reduce NOx and CO emissions to or substantially closeto zero. At the same time, with the operation of the air injectionapparatus 115, management and reduction of the CO emissions is possible.Reduction of the CO emissions requires that the second CO catalystoperate in an oxygen rich environment and, as such, the system 10further includes the air injection apparatus 115 to inject air into thesecond HRSG 80 between the NOx catalyst 110 and the second CO catalyst120 to facilitate CO consumption at the second CO catalyst 120.

Flow to the compressor 21 is controlled by modulation of first inletguide vanes 201 for the gas turbine compressor 21 and second inlet guidevanes 202 for the upstream compressor 30. Control of the flows betweenthese two compressors will also control the flows in the split HRSGarrangement 60.

In accordance with embodiments, the reduction of the NOx and the COemissions is enabled when about 55-65% by mass of the GT engine exhaustis permitted to flow to the first HRSG 70 and about 35-45% by mass ofthe GT engine exhaust is permitted to flow to the second HRSG 80.Particularly, the reduction of the NOx and the CO emissions is enabledwhen about 60% by mass of the GT engine exhaust is permitted to flow tothe first HRSG 70 and about 40% by mass of the GT engine exhaust ispermitted to flow to the second HRSG 80. Of course, it is to beunderstood that this ratio is merely exemplary and could be modified orchanged in response to varying conditions and HRSG or catalystspecifications. These modifications or changes may be predefined or madeon an ad hoc basis by a controller 200 to be described below.

The stoichiometry of the three way catalysts of the second HRSG 80proceeds as follows. The GT engine exhaust includes trace O₂, trace COand trace NOx. The first CO catalyst 100, when it is in use, consumessubstantially all the O₂ so there remains trace CO and trace NOx. In theabsence of substantial amounts of O₂, the NOx catalyst 110 consumes theNOx and some CO to create a GT engine exhaust stream with no O₂, no NOxbut some CO. The air injection provided by the air injection apparatus115 adds enough O₂ to enable the second CO catalyst 120 to consume theCO, leaving no NOx and no CO in the stream but trace O₂ and carbondioxide (CO₂).

A shown in FIG. 2, the system 10 may include controller 200 to modulatethe relative amounts of the portion and the remaining portion of the GTengine exhaust by control of the first inlet guide vanes 201, the secondinlet guide vanes 202 and valve 203 in accordance with sensor 210readings. The valve 203 is disposed on or at the air injection apparatus115 and may be employed to increase or decrease the amount of the airinjected into the second HRSG 80 upstream from the second CO catalyst120. The sensor 210 may be any sensor that is capable of sensing acondition within for example the second HRSG 80 that is reflective of anoperating condition of the GT 20, the second HRSG 80 and/or the threeway catalysts. The sensor 210 may, therefore, comprise a thermocoupledisposed within the conduit 24 or the second HRSG 80 to determine atemperature of the GT engine exhaust or a calorimeter or a gaschromatograph disposed within the second HRSG 80 to evaluate aneffectiveness of the NOx and/or CO consumption, or an Oxygen sensor. Thesensor 210 may be a single component or plural and possibly variouscomponents disposed in various locations.

In operation, the sensor 210 may take measurements of the condition andrelay data to the controller 200 that is reflective of thosemeasurements. Responsively, the controller 200 may send out in real-timecontrol signals to the first inlet guide vanes 201, the second inletguide vanes 202 and the valve 203 that instruct those features to open,close or remain in a current open/close state. Thus, if the exemplary60/40 per cent ratio is currently in effect and it is determined viasensor 210 readings that the NOx and CO emissions are completelyreduced, the controller 200 may control the first and second inlet guidevanes 201, 202 and the valve 203 to continue to remain in their currentopen/closed states. By contrast, if it is revealed that the NOx and COemissions are not completely reduced, the controller 200 may control thefirst and second inlet guide vanes 201, 202 and the valve 203 to modifytheir current open/closed states.

With reference to FIGS. 1 and 3, the system 10 may further include anexhaust system 130, which is coupled to the distal end of the secondHRSG 80 downstream from the second CO catalyst 120. The exhaust system130 directs treated GT engine exhaust to, for example, a stack 300whereby the treated GT engine exhaust is released to the atmosphere. Inaccordance with the description provided above, since the treated GTengine exhaust is free of or substantially free of NOx or CO emissions,the production of electricity provided by the electrical generator 40may be accomplished without significant financial penalty imposed bycurrent regulations governing the emissions of pollutants. However, dueto the reduction of NOx emissions provided for herein, such penaltiesmay be avoided. Moreover, the CO₂ remaining in the treated GT engineexhaust may be collected in a carbon capture system 301 and disposed ofin an environmentally appropriate manner.

In accordance with further aspects and, with reference to FIG. 4, amethod of operating a turbomachine system, such as a gas turbine (GT)engine 20 system, is provided and includes permitting a portion of GTengine exhaust to be received in a first HRSG 70 for delivery to acompressor 21 of the GT engine 20 and a remaining portion of the GTengine exhaust to be received in a second HRSG 80 in which NOx and CO inthe GT engine exhaust are consumed and sensing whether the NOx and theCO consumption is substantially complete. The sensing may beaccomplished by for example measuring the NOx and/or CO and/or O₂emissions (400) or by similarly measuring a temperature of the GT engineexhaust. It may then be determined whether the NOx/CO emissions are zeroor substantially close to zero (401). If they are, the controller 200controls the first inlet guide vanes 201 and the second inlet guidevanes 202 and the valve 203, respectively, to continue to operate intheir current states (410). By contrast, if the emissions are notsubstantially zeroed, the controller 200 adjusts NOx performance 420and/or CO performance 430 as described above. During system 10operation, this feedback control may be consistently repeated inreal-time.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

1-17. (canceled)
 18. A method of operating a turbomachine system togenerate electricity, the method comprising: permitting a portion ofturbomachine exhaust to be received in a first heat recovery steamgenerator (HRSG) for delivery to a compressor of the turbomachine and aremaining portion of the turbomachine exhaust to be received in a secondHRSG in which Oxides of Nitrogen (NOx) and Carbon Monoxide (CO) in theturbomachine exhaust are consumed; sensing whether the NOx and the COconsumption in the second HRSG is substantially complete; and in anevent the NOx consumption is incomplete, modulating the relative amountsof the portion and the remaining portion of the turbomachine exhaustrespectively received in the first and second HRSGs and, in an event theCO consumption is incomplete, modulating an amount of air injected intothe second HRSG.
 19. The method according to claim 18, wherein thepermitting comprises disposing the first and second HRSGs in fluidcommunication with the turbomachine in a split HRSG arrangement.
 20. Themethod according to claim 18, wherein the sensing comprises at least oneof measuring a temperature within the turbomachine and measuring acomposition of the turbomachine exhaust.